1. Field of the Invention
The invention relates to a device that estimates the concentrating distribution of impurities in an anode-side gas channel of a fuel cell that generates electric power by being supplied with a fuel gas on the anode and with air on the cathode.
2. Description of the Related Art
Fuel cell systems that operate with a fuel gas held within an anode-side gas channel in a fuel cell (hereinafter, referred to as the anode dead-end type system) are known as proposed in, for example, Japanese Patent Application Publication No. 2005-353569 (JP-A-2005-353569), Japanese Patent Application Publication No. 2005-327597 (JP-A-2005-327597), Japanese Patent Application Publication No. 2003-317752 (JP-A-2003-317752), Japanese Patent Application Publication No. 2005-353303 (JP-A-2005-353303), Japanese Patent Application Publication No. 9-312167 (JP-A-9-312167), etc. In the anode dead-end type systems, impurity substances, such as nitrogen (N2), water moisture, etc., accumulate in an anode-side gas channel of a fuel cell with the elapse of operation time. If such impurity substances cover the surface of a membrane-electrode assembly (MEA), the electromotive reaction on the electrode catalyst is impeded, resulting in a decline in voltage. Besides, the generated abnormal electric potential may degrade the membrane-electrode assembly (MEA). Therefore, in the related-art anode dead-end type systems, an exhaust valve is opened at an appropriate timing so that the impurity substances accumulated in the anode-side gas channel is exhausted from an end portion downstream of the anode to the outside of the fuel cell system.
In the related-art anode dead-end type system, when the exhaust valve is opened, the fuel gas in the anode-side gas channel as well as the impurity substances is emitted. Therefore, the frequent opening of the exhaust valve brings about degraded fuel economy, and therefore is not preferable. Besides, if the exhaust valve is opened when impurity substances are sufficiently accumulated in a downstream end portion of the anode-side gas channel, the amount of the fuel gas that is wastefully emitted can be correspondingly restrained. Hence, from a viewpoint of improving the fuel economy, it is preferable that the frequency of opening the exhaust valve be restrained as much as possible.
On the other hand, from a viewpoint of maintaining the fuel cell performance, the concentrated accumulation of impurity substances in the downstream end portion of the anode-side gas channel is not preferable. This is because the concentrated accumulation of impurity substances causes decline in the output voltage and degradation of the membrane-electrode assembly as stated above. That is, with regard to the related-art anode dead-end type system, it is demanded to simultaneously accomplish the two contrary tasks, that is, prevention of the decline in the fuel cell performance resulting from accumulation of impurity substances, and the improvement of the fuel economy by restraining the amount of emission of the fuel gas.
However, it is not easy to fulfill the foregoing requirements in the related-art anode dead-end type system. In order to fulfill the requirements, it is necessary to open the exhaust valve according to the situation of accumulation of impurity substances in the downstream end portion of the anode-side gas channel. However, a method for accurately estimating the situation of the accumulation has not bee established. Besides, the estimation of the situation of the accumulation demanded herein is not a mere estimation of the amount of accumulation of impurity substances. In order to reliably emit impurity substances while preventing wasteful emission of the fuel gas, it becomes necessary to estimate where in the anode-side gas channel impurity substances exist and what amount of impurity substances exist, that is, to estimate the concentration distribution of impurity substances in the anode-side gas channel.